Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m–3). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10–2 to 10 μg m–3 are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr–1 of SOA production, or 3.3% of global SOA.

We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the atmosphere.

S1.0 CIMS Sensitivities CIMS sensitivities to the oxidation products were determined in multiple ways. Hydroxyacetone and glycolaldehyde are commercially available and were quantified gravimetrically and by Fourier Transform Infrared Spectroscopy (FT-IR) for CIMS calibration.1 Uncalibrated compounds (glycolic acid and all products identified by m/z) were assigned a generic CIMS sensitivity of 2.5×10 -4 ncts /pptv, and are considered accurate to within a factor of 2. Here, normalized counts (ncts) represent the counts observed at the analyte m/z divided by the reagent ion counts. OH + ISOPOOH à ISOPOOH-OH à ProductsIn Figures Figure S1. The different reaction pathways for the reaction between (1,2)-ISOPOOH and OH radical. Figure S2. The different reaction pathways for the reaction between (4,3)-ISOPOOH and OH radical. 11,12 The general reaction scheme is shown in Figure S3. Figure S3. The reaction scheme as used in the MESMER model (Only an illustration, not the energetically correct picture of the reactions). The ISOPOOH-OH complexes are different for each of the reaction pathways even though they are given with the same energy at this figure. S5 S6 S7In our Mesmer modeling the Lennard-Jones (L-J) parameters of the bath gas were chosen to be a nitrogen gas resembling the atmospheric gas ISOPOOH + OH ISOPOOH-OH complex TS TS TSAbstraction trans-Add1 cis-Add1Add2 trans-IEPOX cis-IEPOX S9We have preformed a sensitivity test of Mesmer input parameters. In our sensitivity test we used three collisional activation/deactivation energies of 50, 100 and 200 cm -1 and two different grain sizes of 25 and 50 cm -1 . We did not observe any significant changes in the reaction rate constants (only changes of a few percent). We have also tested the system with different sizes of grain span, e.g., 10 kT, 20 kT, 30 kT, 40 kT and 50 kT. If a grain span of 30 kT or higher is used, the reaction rate constants do not change. We have therefore used a grain size of 30 kT.The reaction rate constants are sensitive to the choice of the Arrhenius pre-exponential factor (A). Each reaction pathway is a separate Mesmer calculation (See Figure S4 and S5 for the individual reaction pathways) -we have not coupled between the reactions in the fitting of the Arrhenius pre-exponential factor (A). We treat the pre-exponential factor as temperature independent and it is varied between 1.0×10 -12 and 2.0×10 -10 cm 3 molecule -1 s -1 . We use nine different Arrhenius pre-exponential factors to calculate the rates. Three of the factors are from the three reactions of n-butane, 3-methyl-3-butene-1-ol and 1-butene with OH. [14][15][16] The total reaction rate constants (OH +ISOPOOH → Products) of the (1,2)-ISOPOOH and (4,3)-ISOPOOH systems are shown in Table S3 and Table S4, respectively. S3.3 (1,2)-ISOPOOHFor the (1,2)-ISOPOOH + OH reactions, the absolute rate constants of all the different reaction pathways increase with an increase in the Arrhenius pre-exponential factor, and the relative yields (in %) of the reaction pathways also change.The yiel...